Cover Page
The following handle holds various files of this Leiden University dissertation:
http://hdl.handle.net/1887/81578
Author: Swierts, T.
SUMMARY
198
8
SUMMARY
Sponges are the oldest multicellular animal lineage that occurs on Earth today. Despite their special place in evolutionary history, sponges have received relatively little attention in scientific research compared to other ‘charismatic’ organisms such as corals and fish. Among other things, this is due to the logistical problems of working under water and the lack of clear distinctive features between sponge species. However, it is becoming increasingly clear that sponges play a central role in marine ecosystems, such as coral reefs. Sponges, for example, are important in nutrient cycles, such as nitrogen, carbon, silicon and phosphorus. In addition to this, sponges provide habitat for other organisms and can erode and consolidate coral reef structures depending on the species, thereby influencing biodiversity in the system. In view of the changing conditions in oceans and seas, which are related to climate change and other human-induced stressors, it is important to thoroughly understand the role of sponges in coral reefs. This knowledge is essential in order to predict how coral reefs will respond to future conditions and to take appropriate measures to preserve these rich ecosystems. In this thesis, an attempt is made to better understand the evolutionary history of sponges and to gain a better understanding of the cooperation between sponges and the various microorganisms that they harbor. To achieve this goal, the giant barrel sponge has been used as a model organism. This group of sponges is widespread and occurs in the Caribbean and Indo-Pacific, from the east coast of Africa and the Red Sea to Taiwan and New Caledonia. Moreover, giant barrel sponges are abundant components of many coral reefs and are among the largest sponges in existence.
The central questions of this thesis are: 1. How many types of giant barrel sponges exist? 2. What is the evolutionary history of the extant giant barrel sponges? 3. What drives variation in the composition of the prokaryotic community of giant barrel sponges? 4. How do richness, diversity and composition of the prokaryotic communities in (giant barrel) sponges relate to those of other organisms in coral reef ecosystems?
SUMMARY
199
8
already several giant barrel sponges before the Atlantic Ocean and the Indo-Pacific Ocean became separated. Each of these species was split into two populations during the collision of the European and African tectonic plates about 40 million years ago and subsequently developed into different species.
Although the giant barrel sponge species look alike, there are subtle differences in some places. In North Sulawesi (Indonesia), one species has a smooth appearance, while the other is covered with spiky or digitate structures. In Tanzania, one type is bronze-colored and another blue-purple. However, these distinctions are location-specific, and there are no distinctive features that occur ocean-wide. This makes it impossible to distinguish between the species in the field without the ability to perform DNA analysis.
Sponges house a large and diverse community of microorganisms. It is generally believed that these microorganisms play a central role in the physiological processes in the host. Modern DNA-analysis has made it possible to make an extensive screening of the microorganisms that live in giant barrel sponges. At present, millions of microorganisms can be identified from a single individual. It appears that the region in which a giant barrel sponge occurs mainly determines the composition of their microbial community. However, if you look at a smaller scale within a region, for example within one reef or around an island such as Curaçao, it appears that not only is the geographical location important, but the host species to which an individual belongs also plays an important role. The composition of the microbial community in giant barrel sponges is, therefore, dependent on a combination of factors, and the influence of these vary across spatial scales.
